This invention relates to electrically-driven dental injectors for use in injection of an injection liquid such as an anesthetic into the mouth cavity in dental treatment.
A cartridge type injector is generally used in injection of the anesthetic in dental treatment. When the anesthetic is injected into the mouth cavity with such an injector, a dentist must exert a considerable force on the injector to eject the anesthetic from the cartridge through the needle of the injector and also must slowly effect the operation of the injection. For the purpose of solving such a problem, an electrically-driven dental injector has been proposed in recent years. Basically, the electrically-driven dental injector comprises a pinion which is rotated through a transmission gear mechanism by a motor with reduction gear, and a plunger rod having a rack meshed with the pinion. Forward or advance movement of the plunger rod causes a plunger rubber in the cartridge to push, thereby ejecting the injection liquid in the cartridge through a needle penetrated into a rubber plug which seals an end of the cartridge opposite the plunger rubber. In order to inject the injection liquid into the mouth cavity, the plunger rod is advanced at an extremely low speed. Upon completion of the operation of injection, it is required to retract the plunger rod from the advanced position, but if the retraction of the plunger rod should be effected with a reverse rotation of the motor with reduction gear, it would take too much time to return the plunger rod from it""s advanced position to its initial position. In order to solve this problem, the operative connection between the plunger rod and its drive mechanism may be interrupted so that the plunger rod can be manually freely pushed back.
However, since there has heretofore been employed an arrangement for disengaging the pinion from the mesh with the rack to interrupt the operative connection between the plunger rod and the drive mechanism, it has been necessary for the dentist to manually operate an operating part from the exterior of the injector. It has been very troublesome to manually push back the plunger rod while operating the operating part each time. There has been strong needs for development of an electrically-driven dental injector wherein a plunger rod can be manually pushed back directly upon completion of the operation of injection without any manual operation of the injector.
An object of the invention is to provide an electrically-driven dental injector which can comply with such needs.
According to the present invention, there is provided an electrically-driven dental injector comprising a housing having a nose portion, a support member secured to said nose portion and having a forwardly protruding socket for receiving a barrel adapted to contain a cartridge therein, means for coupling the barrel with the socket, a plunger rod sidably mounted in a bore in the support member and movable between its initial position and it""s forwardly advanced position to push a plunger rubber in the cartridge, the plunger rod having a rack, a pinion meshed with the rack to advance the plunger rod, an electrical motor with reduction gear mounted within the housing, a planetary reduction gear located between said motor and said pinion and housed in a casing, a latch mechanism for coupling a ring gear of the planetary reduction gear with the casing or uncoupling the ring gear from the casing to transmit drive rotation of the motor to said pinion or interrupt the transmission of the rotation between the motor and the pinion, and an operating rod extending through the support member and operatively cooperating with the latch mechanism, whereby when the plunger rod is manually pushed from it""s advanced position toward its initial position, the latch mechanism is released and at the same time, said operating rod protrudes beyond the inner bottom surface of the socket, and when the barrel is coupled to the socket, it pushes back the operating rod barrel with an end face of the barrel to engage the latch mechanism.
In a preferred embodiment of the invention, the planetary reduction gear comprises a two stage planetary reduction gear including two planetary reduction gear units, the ring gear being of the first planetary reduction gear unit. The latch mechanism includes a plurality of latching grooves formed in the ring gear on its periphery with a lug defined between the two adjacent grooves, a steel latching ball located in an aperture in the peripheral wall of the casing and adapted to engage or disengage from the latching groove and a lever pivoted at its lower end to housing adjacent the casing, the forward end of the lever operatively cooperating with the operating rod. A length of said operating rod is such that the forward end of the operating rod is retracted to the inner bottom surface of the socket when it abuts and pushes the lever toward the latching ball to force it into the latching groove, thereby coupling the ring gear with the casing.